Effect of oxidative aging of biochar on relative distribution of competitive adsorption mechanism of Cd2+ and Pb2+

In this study, aged biochar (CCB350 and CCB650) were obtained from pyrolysis of corn stalk biochar (CB350 and CB650) at the degree of 350 °C and 650 °C by artificial oxidation with hydrogen peroxide (H2O2). Also, the mechanism of Pb2+ and Cd2+ on fresh and aged biochars was analyzed qualitatively and quantitatively by batch adsorption experiments combined with characterization. The adsorption isotherm results showed that aging treatment decreased the adsorption capacity of Pb2+ and Cd2+ and inhibited the competitive adsorption behavior of heavy metals. In the single-metal system, precipitation and cation exchange were considered as the main adsorption mechanisms for CB350 and CB650, with a ratio of 40.07–48.23% and 38.04–57.19%, respectively. Competition between Pb2+ and Cd2+ increased the relative contribution of mineral precipitation, but decreased the contribution of cation exchange mechanism. Aging resulted in the rise of the contribution of surface complexation to the adsorption of Pb2+ and Cd2+ on biochars, especially in low-temperature biochars, but weakened the contribution of mineral precipitation to the adsorption. Further, the contribution of other adsorption mechanisms was significantly enhanced for high-temperature aged biochars. These results are important to evaluate its long-term application prospects in the natural environment.


Materials and methods
Preparation of biochars. Samples of corn straw used to produce biochar were collected from Baotou, Inner Mongolia, China. The preparation method for fresh biochar (CB350 and CB650) is provided in the supplementary materials.
Aged boichars were prepared as described by Nie et al. 35 . Briefly, the fresh biochars (10 g) and 20% H 2 O 2 (200 mL) were mixed with the solid-liquid ratio of 1:20 (w/v) in a plastic bottle of 250 mL, and the samples were presented in a reciprocating shaker at 24 h room temperature. In the process of oxidation, the bottles were opened repeatedly to release air. After aging, the samples were filtered by centrifugation and rinsed several times. And the residual H 2 O 2 was removed with distilled water. The aged biochars were dried at 80 °C and labeled as CCB350 and CCB650, respectively. The surface characteristics of the biochars were also analyzed using SEM-EDS, FTIR, XPS, XRD, and BET. The ash content, pH, surface zeta potential and total C, H, O, N and S contents of biochars were measured. The details were described in the supplementary materials.

Batch adsorption experiments.
A certain amount of Cd(NO 3 ) 2 ·4H 2 O and Pb(NO 3 ) 2 were dissolved in the background solution (0.01 mol/L NaNO 3 ) to prepare Cd 2+ (1000 mg/L) and Pb 2+ (1000 mg/L). 0.1 g biochar (CB350, CB650, CCB350 and CCB650) was mixed with 50 mL Cd 2+ or Pb 2+ solution for adsorption isotherm experiment. The mass ratio of Cd 2+ to Pb 2+ was chosen as 1:1 in the binary system. The initial pH of the solution was adjusted to 5.0 ± 0.1 by 0.1 mol/L NaOH or 0.1 mol/L HNO 3 . This pH ensured the maximum adsorption capacity of biochar while avoiding the effect of Pb 2+ or Cd 2+ adsorption due to precipitation. We added 0.01 mol/L of NaNO 3 to all the solutions as background electrolyte. The concentration of Cd 2+ and Pb 2+ in both the single and binary metal systems range from 0 to 200 mg/L. The biochar and different concentrations of Cd 2+ or Pb 2+ solutions were then shaken in an HY-4 speed-controlled multipurpose shaker (Jintan Ronghua Instrument Manufacturing Company, Jinhua, China) at 200 rpm and 25 ± 1 °C for 2880 min.
As for adsorption kinetics, 0.1 g of biochars (CB350, CB650, CCB350, and CCB650) were mixed with 50 mL of 100 mg/L for single Cd 2+ , single Pb 2+ and binary Cd 2+ and Pb 2+ at room temperature, and shaken at 200 rpm for 2880 min.
Finally, the suspension was centrifuged at 4000 rpm for 20 min and filtered by a 0.45 μm microporous filter. The concentrations of Cd 2+ and Pb 2+ in the filtrate were determined by flame atomic absorption spectrophotometry (Ice 3500, Thermo Fisher Scientific, USA). The adsorption amount of heavy metals on biochar (Q e ) was calculated as follows: where Q e (mg/g) is the adsorption capacity, C 0 (mg/L) and C e (mg/L) are the initial and equilibrium concentration, respectively. V (L) is the volume of mixed solution, and m (g) is the dry mass of biochar.
The adsorption isotherms were simulated using Langmuir, Freundlich and Langmuir-Freundlich models, and the adsorption kinetics were fitted using Lagergren's pseudo-first-order (PFO), pseudo-second-order (PSO) and intra-particle diffusion (IPD) kinetic equations (see Supplementary Material). Inherent minerals were removed by rinsing with 1.0 mol/L HCl to obtain demineralized biochars. And the untreated and demineralized biochars were mixed with solutions of Pb 2+ and Cd 2+ 200 mg/L. After adsorption, the solution concentration was measured. The fresh biochar was then mixed with 0.01 mol/L NaNO 3 (blank) and www.nature.com/scientificreports/ the pore structure of the biochar became smoother and clearer after ageing treatment. Furthermore, the SSA of CB350 and CB650 also increased by 43.59% and 32.12% after chemical oxidation, which could be illustrated by the finding that the oxidation process was usually accompanied by a more intense decomposition and consumption of organic matter, which led to the expansion of biochar pores and the creation of more pore structures 44 . Further, the reduction of impurities washed away by the water bath heating process led the pore structures to become smoother 45 , which was confirmed by the results of SEM images. The similar finding was evaluated by Mia et al. 46 , believing that the SSA of biochar rose after the oxidation of hydrogen peroxide. With the exception of CB650, when the pyrolysis temperature was increased, the C content of the biochar rose, while the O, H and N content were on the decrease. The contents of O were higher for the aged biochars than the fresh biochars, but the contents of C were lower. The changing of C and O contents indicated that aging may decompose unstable aliphatic carbon in biochar and introduce more oxygen functional groups, resulting in a fall in C and an obvious rise in O 47 . For CB650, the C content increased by 7.32% after chemical oxidation treatment, which may be due to the mass loss caused by the activation of own oxygen and ash outweighing the oxidation loss of more resistant matrix carbon 48 . In general, O/C, H/C and (O + N)/C are used as evaluation indexes of hydrophilicity, aromatics, and polarity of biochar, respectively 49 . After chemical oxidation, the O/C values of CB350 and CB650 increased by 86.27% and 38.89%, respectively, indicating that chemical oxidation enhanced the hydrophilicity of biochar. At the same temperature, the low (O + N)/C meant that there were relatively fewer polar functional groups on the surface of fresh biochar.
Surface functional groups and crystalline minerals in the biochar were characterized by FTIR and XRD, as shown in Fig. S2. FTIR results showed that CCB350 had stronger intensity of bands at 3414 cm −1 and 1690 cm −1 compared to CB350, indicating that CCB350 exhibited a higher abundance of -OH and -COOH. CCB650 showed a more significant enhancement of the C-O-C stretching vibration peak at 1021 cm −1 than CB650. In summary, the aging treatment induced more O-containing functional groups. The XRD pattern showed that the main crystalline form on the surface of fresh biochar was KCl, and the KCl peak weakened or even disappeared after chemical aging, probably because of the high solubility of KCl in water.
In order to analyze the effects of aging treatments on the oxygen-containing functional components on the surfaces of biochar, XPS analysis was further performed. The fitted spectra of fresh and aged biochars are shown in Fig. S3. The main C1s peaks of biochars were divided into four peaks: C-C, C-O, C=O, and O=C-O 19 . In fresh biochars, the content of C-C increased from 59.04 to 70.24% with the increase of pyrolytic temperature. This indicated that the increase of pyrolytic temperature could reduce the content of oxygen-containing functional groups on the surface of biochar. After chemical oxidation aging, the C-O contents of CB350 and CB650 increased by 0.32% and 5.02%, respectively. The C=O contents of CB350 and CB650 increased from 6.24% and 14.62% to 14.91% and 17.47%, respectively. Overall, biochars exhibited an increasing trend for the oxygenated functional groups.
Effects of aging on the adsorption isotherms of Pb 2+ and Cd 2+ by biochars. The adsorption isotherms of Pb 2+ and Cd 2+ on fresh and aged biochars are shown in Fig. 1. The adsorption abilities of them on all biochars increased rapidly with increasing heavy metal concentrations at the initial stage. After that, it inclined to increase slowly and gradually saturate, except for the Pb 2+ adsorption isotherms on CB650, indicating that the surface absorbing sites on CB650 were not totally covered by Pb 2+ . In the binary metal system, the presence of coexisting ions significantly altered the adsorption isotherms of them. The adsorption isotherms in binary metal systems had a shorter initial linear part and could reach equilibrium at lower concentrations. Therefore, this indicated that the presence of competitive ions achieved the effect of completely occupying the adsorption sites of biochar 50 . Aging treatment also had an effect on the sorption isotherms of Pb 2+ and Cd 2+ . The adsorption capacity of aged biochar to heavy metals was lower than that of fresh biochar.
The adsorption isotherms were suitable for the Langmuire, Freundlich and Langmuir-Freundlich models in both metal systems. The fitted parameter values are shown in Table S2. All fresh biochar adsorption isotherms of Pb 2+ and Cd 2+ were better fitted by the Langmuire model (R 2 = 0.9490-0.9822). Most of the adsorption isotherms for the aged biochars were better fitted by Langmuir-Freundlich model, which suggested that the adsorption of metal ions on biochar after aging transformed from uniform adsorption to heterogeneous surface monolayer adsorption 23 . That is, it was close to the Freundlich isotherm at low concentrations of metal ions, while it described monolayer adsorption in a similar way to the Langmuir isotherm at high concentrations [ 51 ]. The Q m of CB350 and CB650 for Pb 2+ absorption using the Langmuir model were 84.91 mg/g and 149.98 mg/g, better than that of Cd 2+ on CB350 and CB650 (35.66 mg/g and 65.43 mg/g, respectively). Competition affected the adsorption on biochars, the Q m values of Cd 2+ on CB350, CCB350, CB650, and CCB650 decreased by 49.99%, 43.97%, 62.11%, and 20.66%, respectively. The Q m values of Pb 2+ on CB350, CCB350, CB650, and CCB650 decreased by 42.45%, 20.73%, 25.91%, and 18.33%, respectively. The decrease in Q m values was greater for Cd 2+ adsorption than for Pb 2+ under competitive conditions. Therefore, compared to Cd 2+ , Pb 2+ was more competitive.
Moreover, the ratio of a*(Q e of Pb 2+ or Cd 2+ in coexisting systems)/a (Q e of Pb 2+ or Cd 2+ in single metal system) could also be used to indicate the degree of reduction in the adsorption capacity of Pb 2+ or Cd 2+ because of the competition for sites of adsorption in coexisting systems 52,53 . We evaluated the a*(Pb 2+ )/a (Pb 2+ ) ratios for CB350, CCB350, CB650, and CCB650 were 0.54, 0.75, 0.95, and 0.85, respectively, while the a*(Cd 2+ )/a(Cd 2+ ) ratios were 0.57, 0.68, 0.41, and 0.66, respectively. The a*(Pb 2+ )/a(Pb 2+ ) ratios were greater than the a*(Cd 2+ )/ a(Cd 2+ ) ratios for all biochars except CB350, showing that the adsorption of Cd 2+ was more influenced by Pb 2+ . In particular, the ratio a*(Pb 2+ )/a(Pb 2+ ) (0.95) was close to 1, indicating that the existence of Cd 2+ had limited impact on the adsorption of Pb 2+ on CB650. This may be due to the following reasons: (1) compared to Cd 2+ (4.26 Å), Pb 2+ has a tinier hydrated radius (4.01 Å), and (2) 13 . Therefore, Pb 2+ has stronger affinity for most functional groups and occupies more internal complexing sites in the coexistence system 54 .
After aging, the ratio of a*(Pb 2+ )/a(Pb 2+ ) for CCB650 decreased to 0.85, while the ratio of a*(Cd 2+ )/a(Cd 2+ ) increased to 0.66, indicating that aging inhibited the competitive adsorption of Pb 2+ and Cd 2+ by CB650. The competitive adsorption performance of low-temperature biochar was completely different from high-temperature biochar. The values of a*(Cd 2+ )/a(Cd 2+ ) and a*(Pb 2+ )/a(Pb 2+ ) on the fresh biochar (CB350) were close, while the a*(Cd 2+ )/a(Cd 2+ ) and a*(Pb 2+ )/a(Pb 2+ ) ratios of CCB350 grew to similar levels after aging. The above results indicated that there was no significant competitive adsorption in fresh and aged low temperature biochars. The Q m of biochars decreased after aging treatment, except for Cd 2+ adsorption capacity of CCB350 in the binary metal system. This showed that the adsorption capacity had a tendency to decrease for most biochars when aged with H 2 O 2 . A similar trend was previously reported 35,55 . This might be due to the fact that aging introduced more O-containing functional groups, which could connect neighboring water molecules via strong H-bonding to form three dimensional water clusters. Thus, these clusters impeded the combination of heavy metal irons and functional groups on the surface of the biochar 56 .

Effects of aging on the adsorption kinetics of Cd 2+ and Pb 2+ by biochars. Adsorption kinetics of
Pb 2+ and Cd 2+ on fresh and aged biochars in the systems is shown in Fig. 2. It was evident that Pb 2+ and Cd 2+ adsorption on biochars rose sharply in the first 0-120 min, rose at a slow level for the next 120-720 min, and reached equilibrium in 720-2280 min. The adsorption process has three stages: (1) the rapid adsorption process, (2) the steady adsorption process, and (3) the process in equilibrium. The multi-linearity of the intra-particle diffusion plot suggested that the whole adsorption process was composed of three stages (Fig. S4). In the first stage, instantaneous adsorption resulted in the sharper portion in the graph. The Pb 2+ and Cd 2+ transferred from the aqueous phase to the external biochars surface. In the second step, there was internal diffusion of Pb 2+ and Cd 2+ through the pores of biochars. It showed a decrease in the slope due to the gradual decrease in the adsorption rate. The third region was the final equilibrium stage. The whole adsorption process was controlled by a rapid first stage, a slower second stage, and a much slower third stage until reaching equilibrium. At the end of the fast phase of adsorption, the adsorption amounts of Cd 2+ and Pb 2+ by biochars accounted for about 84.60-97.57% www.nature.com/scientificreports/ of the total adsorption amounts. These results showed that the rapid stage in adsorption was more prominent in this process. Aging treatment was without vital effect on the equilibrium time of heavy metals in adsorption. The data were fitted by the model of pseudo-first-order (PFO) and pseudo-second-order (PSO), and the relevant parameters are shown in Table S3. The coefficient of determination (R 2 ) obtained from the pseudo-firstorder models (0.5617-0.9424) was lower than that of the pseudo-second-order model (0.9050-0.9786) for all biochars. Pb 2+ and Cd 2+ adsorption on fresh and aged biochars was more consistent with the pseudo-secondorder model, suggesting that chemisorption (ion exchange, complexation, and precipitation) was the main rate-limiting step in this adsorption process 57 . Moreover, the calculated values (Q e ) of the pseudo-second-order model were closer to the values of experiment (Q exp ).
To further investigate the rate limiting steps involved in the adsorption process, the intra-particle diffusion (IPD) model was adopted in a kinetics analysis. In this study, data for the Cd 2+ and Pb 2+ adsorption on the fresh and aged biochars in the single and binary systems within 720 min were well fitted by the IPD model (Table S4), suggesting that intra-particle diffusion played a significant role in Cd 2+ and Pb 2+ adsorption. The sorption rate in stage 1 (k i1 ) was much higher than that in other stages (k i2 and k i3 ), reflecting the external diffusion was the rate-determining step during sorption. Diffusion resistance was evaluated by the intercept C of each stage, which provided information about the thickness of the boundary layer 58 . The intercept C of each stage followed the order of C 1 (first stage) < C 2 (second stage) < C 3 (third stage). It indicated that external diffusion resistance was the lowest in early stages.

Effects of aging on the adsorption mechanisms of Pb 2+ and Cd 2+ by biochars. Cation ex-
change. Many cations are remained on the surface of the biochar by direct electrostatic attraction, complexation or precipitation. In the process of the adsorption, these cations on the biochar are in exchange for Cd 2+ and Pb 2+ in solution through a cation exchanging reaction 20,39 . To illustrate this possible mechanism, the contents of cations in the solutions before and after adsorption of Pb 2+ and Cd 2+ by biochar (CB350, CB650, CCB350, and CCB650) were determined by SEM-EDS (Fig. 3). The percentage contents of Pb 2+ and Cd 2+ elements on the surface of biochars increased after adsorption, while the percentage contents of the metal elements (K + , Ca 2+ , Na + and Mg 2+ ) decreased or even lowered to the limit of detection. To further quantify the amount of the www.nature.com/scientificreports/ metals from the solution before and after the adsorption of Pb 2+ and Cd 2+ by biochar in single and binary metal systems, we calculated the net release amount of metal ions (Fig. 4). After adsorption of Pb 2+ and Cd 2+ , the net release amounts of the elements from the single and binary metal systems were 0.0196-0.2385, 0.0806-0.4648, 0.0333-0.1550, and 0.0639-0.2454 meq/L, respectively. The net emissions of Ca 2+ and Mg 2+ were much greater than those of K + and Na + , indicating a vital role in the cation exchange process. This was mainly because K + and Na + could only be retained on the surface of biochars by external spherical electrostatic attraction to the negatively charged position on the surface. However, Ca 2+ and Mg 2+ remained on biochar due to precipitation (CaCO 3 or MgCO 3 ) or complexation of inner sphere for the acid groups on the surface of biochar 59 .
In comparison with the fresh biochars, the total release amounts of K + , Ca 2+ , Na + and Mg 2+ of the metal systems obviously decreased after absorbing the Pb 2+ and Cd 2+ by aged biochars, which may be related to the leaching of soluble salt ions during the aging process 60 .
Complexation with oxygen-containing functional groups. The oxygenated groups (e.g., -OH, -COOH, -R-OH) distributed on surfaces of the biochar formed metal complexes with heavy metal ions, which reduced the concentration and mobility of heavy metals 61 . A lot of studies have revealed that the complexation between heavy metals and oxygenated functional groups is a significant mechanism for biochar adsorption 20,39,62 . The changes of these groups in biochar adsorption were analyzed by FTIR (Fig. 5). The dominant bands at 3414 cm −1 , 2932 cm −1 , 1701-1680 cm −1 , 1564-1597 cm −1 and 1021-1084 cm −1 represented -OH, C-H of aromatic groups, -COOH, C=O/C=C, and C-O-C, respectively 23,61 . These functional groups were transformed after the adsorption. For CB650, the intensity of the stretching vibration peak of -OH did not change evidently after the adsorption of Pb 2+ and Cd 2+ , which was due to the low content of -OH in the high-temperature biochar. However, the intensity and peak position of -OH changed after the adsorption by CCB350 and CCB650, indicating that -OH was in the process of the adsorption of these metals 28 . The intensity of aromatic C-H weakened after the heavy metal adsorption by CB350 and CCB350, while CCB650 + Pb + Cd was enhanced, indicating that the aromatic group C-H was in the adsorption of Pb 2+ and Cd 2+63 . The strength of the -COOH stretching vibration peaks of CB350 + Pb, CB350 + Cd and CCB350 + Pb + Cd weakened after the adsorption of the heavy metals Pb 2+ and Cd 2+ . The band at 1441-1437 cm -1 was classified to CO 3 2− , at 1104 cm −1 was because of the P-O stretching vibrations 64 , which may be due to the metallic carbonate and metallic phosphate precipitates formation, respectively. The C=O/C=C intensities of CB650 + Pb, CB650 + Cd, CB650 + Pb + Cd, CCB650 + Pb, and CCB650 + Cd were weakened. The stretching vibration peak of C-O-C showed a small shift and change. These results indicated that these groups were in the adsorption of Pb 2+ and Cd 2+ by biochar. ) can also be precipitated with heavy metal irons 65 . The function of these anions in the adsorption of Pb 2+ and Cd 2+ on biochar has been reported by Fei et al. 66 . In the present study, after adsorbing heavy metals, SEM results showed that white granular crystals were formed on the surface of biochar. Furthermore, EDS analysis believed that the www.nature.com/scientificreports/ white granular crystals involved Cd and Pb. Hence, the ability of mineral precipitation to adsorb heavy metals cannot be ignored.
To further determine their precipitate crystals, all biochars before and after the adsorption of Pb 2+ and Cd 2+ were scanned by XRD (Fig. 6). The XRD patterns also showed the presence of CdCO 3 , PbCO 3 , Pb 3 (CO 3 ) 2 (OH) 2 , and PbSe on the surface of the biochars after adsorption. Biochar is rich in inorganic minerals. The higher the ash contents of biochar, the stronger the precipitation of heavy metals 67 . The ash contents of biochar at high temperature (CB650 and CCB650) were higher than those of low-temperature biochars (CB350 and CCB350). Similarly, the ash content of the fresh biochars (CB350 and CB650) was greater than that of the aged biochars (CCB350, CCB650). This showed that a low pyrolysis temperature and aging could reduce the precipitation of heavy metals.
Other potential mechanisms. Except for cation exchange, complexation, and precipitation, there are other adsorption mechanisms 68 . Except for functional groups (-OH, -COOH, -R-OH), the influence of other functional groups (γ-CH, C=C) and basic groups (C=N, -NH 2 ) on the adsorption of Pb 2+ and Cd 2+ by biochar cannot be neglected 38 . More aromatic structures were formed in the biochar with a rising pyrolysis temperature, which could allow the donation of π-electron to interact with Pb 2+ and Cd 2+ , resulting in an excellent concordant affinity for heavy metals 69 . Figure 5 showed that in comparison with low-temperature biochar (CB350, CCB350), the high temperature biochar (CB650, CCB650) caused more changes in aromatic C-H functional groups at 400-700 cm -1 after adsorption, indicating that more aromatic functional groups, such as the γ-CH of furan and the β-ring of pyridine. This was similar to previous results 70,71 .
Furthermore, biochar is able to adsorb heavy metal cations (Cd 2+ , Pb 2+ , Zn 2+ , Cu 2+ , Ni 2+ , etc.) due to its negative surface charge 72 . To elucidate the role played by electrostatic attraction under experimental conditions, the surface zeta potential values of CB350, CB650, CCB350 and CCB650 were examined at pH 5. The zeta-potential values for CB350, CB650, CCB350 and CCB650 were − 53.5 mV, − 45.5 mV, − 57.5 mV and − 56.4 mV, respectively, indicating that all biochar surfaces were negatively charged under these experimental conditions. Therefore, electrostatic attraction was expected to generate between positively charged metal cations and negatively charged functional groups.   72,73 . In this experiment, the SSA of high temperature and aged biochar was greater than that of low temperature and fresh biochar, so physical adsorption might play a major role in high temperature and aged biochar.
Effects of aging on the quantitative contribution of adsorption mechanisms by biochars. The contribution of cation exchange (Q ce ), precipitation with minerals (Q pre ), surface complexation (Q com ) and other mechanisms (Q oth ) within the fresh and aged biochars to the adsorption of the metals in the systems were calculated (Fig. 7). Cation exchange was the major adsorption mechanism for the fresh low-temperature biochar (CB350) in the single system. The values of Q ce for Cd 2+ and Pb 2+ reached 20.02 and 30.89 mg/g, accounting for 48.23% and 40.07% of the total adsorption capacity, respectively. The contribution of Q ce was going down with the increase of pyrolysis temperature. This might be due to the fact that mineral crystallization inhibits the release of cations at high pyrolysis temperatures 74 . The main adsorption mechanism of CB650 in the single system changed from cation exchange to mineral precipitation. The Q pre of Cd 2+ and Pb 2+ were 32.84 and 36.76 mg/g, corresponding to 57.19% and 38.04% of the adsorption contribution, respectively. Under competitive conditions, the Q ce /Q t values were at a lower level, compared to those in single metal systems. Precipitation with minerals formed the biggest fraction for CB350 and CB650 in the binary metal system, which contributed 44.36% and 49.20%, respectively.
After aging, the values of Q com and Q com /Q t for CCB350 and CCB650 increased to varying degrees under both single and binary metal systems, and this phenomenon was more obvious for low-temperature biochar. For CB350, the Q com /Q t values of Cd 2+ and Pb 2+ were 2.12% and 0.28%, respectively. However, after aging, the Q com /Q t values of Cd 2+ and Pb 2+ showed 10.20% and 0.53%, respectively, which increased by 8.08% and 0.25%, respectively. Further, the Q pre and Q pre /Q t values of CCB350 and CCB650 decreased significantly compared with fresh biochars in all systems, and the Q oth /Q t values of CCB650 increased significantly after aging.
The above results showed that cation exchange (Q ce ) belonged to the main mechanism for CB350 and CCB350 (low temperature biochar) in a single-metal system, with the Q ce /Q t values ranging from 35.63% to 48.23%. Precipitation with minerals (Q pre ) and other mechanisms (Q oth ) were foremost for CB650 and CCB650 in the single-metal system, respectively. The competitive interaction between Cd 2+ and Pb 2+ increased the relative contribution of mineral precipitation. Aging resulting in the contribution of surface complexation to Cd 2+ and

Conclusions
The aging of corn straw biochar significantly changed its physicochemical characteristics, affecting the competitive adsorption behavior and mechanism of Cd 2+ and Pb 2+ . In the single-metal system, cation exchange was the key adsorption mechanism for biochar (35.63-48.23%) at a low temperature. However, mineral precipitation dominated Cd 2+ and Pb 2+ adsorption (38.04-57.19%) on fresh high-temperature biochars. Competitive interactions between Cd 2+ and Pb 2+ increased the relative contribution of mineral precipitation, but the contribution of cation exchange mechanism was reduced. Biochar aging increased the contribution of surface complexation to the adsorption of Cd 2+ and Pb 2+ on biochars, especially in low-temperature biochars, but weakened the contribution of mineral precipitation to the adsorption. Further, the contribution of other adsorption mechanisms was significantly enhanced for high-temperature aged biochars. Therefore, the results in this study are expected to provide a theoretical basis for the long term application of biochar in the remediation of heavy metal compound polluted soil.

Data availability
The data used to support the findings of this study are included within the article and the supplementary information files.